The invention relates to ventilation of a turbine key and keyway and, more particularly, is generally applicable to ventilation of axial keys and keyways used to secure turbine wheels to a rotatable shaft.
Some steam turbines utilize such large rotors that the turbine wheels, which carry turbine blades, or buckets, at their radially outer portions, are not an integral part of the shaft of the rotor. Each wheel of such turbines typically includes a hub section disposed generally at the radially inner portion of the wheel and each hub section includes a bore therethrough for receiving and circumferentially surrounding the shaft of the turbine.
To ensure proper and efficient operation of the turbine, it is required that turbine wheels be disposed at substantially fixed circumferential and axial locations relative to the shaft and relative to other wheels on the shaft. A wheel that is not an integral portion of the shaft may be secured to the shaft by an interference shrink fit between the radially inner surface of the hub defining the wheel bore and a cooperating surface of the shaft. Additionally, in order to decrease the likehood of axial and/or tangential (i.e. circumferential) motion of a wheel with respect to the shaft and/or to other wheels on the shaft, a key and an accommodating keyway may be used to secure the wheel to the shaft in order to augment the restraining capability of the interference shrink fit.
It has been suspected that stresses in the wheel hub due in part to the key, keyway and shrink fit, in combination with other stresses generated by normal operation of the turbine, e.g. thermal stress and load cycling, or adverse operational conditions, e.g. introduction of contaminants in the steam, may create an environment in the vicinity of the hub of the wheel which is conductive to initiating and fostering stress corrosion. The precise mechanism which produces stress corrosion is not fully understood, however, it is believed that if accumulation of water, such as is obtained from condensed steam, is minimized, along with minimizing the concentration of oxygen and other non-condensible gases, such as carbon dioxide, in steam and/or in water which contacts the hub region of the wheel, including the key and keyway, then the probability of stress corrosion occurring will be reduced, if not eliminated. Since it is crucial that steam follow the designed steam flow path through a turbine in order to obtain maximum overall turbine efficiency, any attempt to alleviate the aforementioned problems should do so with minimum interference with the main, or working, steam flow path.
Accordingly, it is an object of the present invention to provide means and method for reducing and/or eliminating build-up of oxygen and other non-condensible gas concentration in the hub area, especially in the region of the key and keyway of a turbine wheel that is not an integral part of the shaft.
It is another object of the present invention to provide means and method for reducing accumulation of water in the hub region of a turbine wheel, especially in the region of a key and keyway, wherein the wheel is not integral with the turbine shaft, and is secured to the shaft, at least in part by the key disposed in the cooperating keyway.
Still another object of the present invention to provide means and method for reducing accumulation of water in the hub region of a turbine wheel without affecting steam flow through the desired working steam flow path of the turbine.